A series of mysterious murders, committed by the poison of a black Mamba, puts London in distress. Scotland Yard's Superintendent Elford suspects that the "Squeaker" is behind the murders - the mysterious stranger, the underworld is afraid of and is unsuccessfully hunted by Scotland Yard. The clever mugger grabs his share of each robbery's loot, in return for not reporting the burglars to the police. Superintendent Elford's investigations lead him to a well-known animal wholesale business, where a Mamba was stolen. The former shopkeeper was blackmailed by the "Squeaker" and subsequently forced to death. His widow, the charming old Mrs. Mulford, now looks after dismissed convicts and has handed over the business to the obscure Frank SutSound. The suspicion focuses more and more on the sly animal keeper Krishna. But one morning he also disappears...